This invention relates to apparatus for imaging a body and, more particularly, to an apparatus and method which employs an improved technique of dynamic focusing. The technique is particularly applicable to ultrasonic imaging systems.
During the past two decades ultrasonic techniques have become more prevalent in clinical diagnosis. Such techniques have been utilized for some time in the field of obstetrics, neurology and cardiology, and are becoming increasingly important in the visualization of subcutaneous blood vessels including imaging of smaller blood vessels.
Various fundamental factors have given rise to the increased use of ultrasonic techniques. Ultrasound differs from other forms of radiation in its interaction with living systems in that it has the nature of a mechanical wave. Accordingly, information is available from its use which is of a different nature than that obtained by other methods and it is found to be complementary to other diagnostic methods, such as those employing X-rays. Also, the risk of tissue damage using ultrasound appears to be much less than the apparent risk associated with ionizing radiations such as X-rays.
The majority of diagnostic techniques using ultrasound are based on the pulse-echo method wherein pulses of ultrasonic energy are periodically generated by a suitable piezoelectric transducer such as a lead zirconate-titanate ceramic. Each short pulse of ultrasonic energy is focused to a narrow beam which is transmitted into the patient's body wherein it eventually encounters interfaces between various different structures of the body. When there is a characteristic impedance mismatch at an interface, a portion of the ultrasonic energy is reflected at the boundary back toward the transducer. After generation of the pulse, the transducer operates in a "listening" mode wherein it converts received reflected energy or "echoes" from the body back into electrical signals. The time of arrival of these echoes depends on the ranges of the interfaces encountered and the propagation velocity of the ultrasound. Also, the amplitude of the echo is indicative of the reflection properties of the interface and, accordingly, of the nature of the characteristic structures forming the interface.
There are various ways in which the information in the received echoes can be usefully presented. In one common technique, the electrical signals representative of detected echoes are amplified and applied to the vertical deflection plates of a cathode ray display. The output of a time-base generator is applied to the horizontal deflection plates. Continuous repetition of the pulse/echo process in synchronism with the time-based signals produces a continuous display, called an "A-scan," in which time is proportional to range, and deflections in the vertical direction represent the presence of interfaces. The height of these vertical deflections is representative of echo strength.
Another common form of display is the so-called "B-scan" wherein the echo information is of a form more similar to conventional television display; i.e., the received echo signals are utilized to modulate the brightness of the display at each point scanned. This type of display is found especially useful when the ultrasonic energy is scanned transverse the body so that individual "ranging" information yields individual scanlines on the display, and successive transverse positions are utilized to obtain successive scanlines on the display. This type of technique yields a cross-sectional picture in the plane of the scan, and the resultant display can be viewed directly or recorded photographically or on magnetic tape. The transverse scan of the beam may be achieved by a reflector which is scanned mechanically over a desired angle.
In systems of the type described, the transducer is of finite size, and the beam transmitted and/or received by the transducer has a finite cross section which is a limiting factor on the resolution capabilities of the imaging system. It is known that the ultrasound beam can be "focused," by providing a suitable lens, such as is described in the U.S. Pat. No. 3,958,559, and/or by segmenting the transducer and coupling the different transducer segments to the transmitter/receiver circuitry via different delays. One can readily visualize the focusing effect of the segmented transducer in conjunction with different delays by observing that the ultrasound path to or from a given focal point to each of a plurality of concentric transducer segments is different for each segment. Typically, the geometrical path between the center transducer segment and the focal point is shortest and the geometrical path between the focal point and the outer transducer segment is longest, with the path to each intermediate transducer segment depending upon its size and relative position in the order of segments. Accordingly, ultrasound energy transmitted from the center segment would generally arrive at the focal point before the beam energy transmitted from the outer transducer segments and, similarly, an ultrasound echo reflected from the focal point will return sooner to the center transducer segment than to the outer transducer segments. Focusing is thus generally achieved by providing appropriately longer delays (typically, but not necessarily, electronic delays) in conjunction with the central segments of the transducer than are provided for the outer segments thereof.
It is also presently known in the art that the required delays vary as the focal point under consideration varies, as would typically be the case in a pulse echo system wherein information is to be received over a range of depths in the body being investigated by the ultrasound beam. In such instance, it is recognized that using fixed delays the beam is only "focused" at one particular focal length (or depth range), and the different geometries associated with other depths in the body require other delays to achieve an optimum focus at each point. Briefly, this can be visualized by recognizing that as the focal point moves deeper into the body, the difference between arrival times at the various transducer segments becomes less and less. Accordingly, a "dynamic focus" can be achieved (during receiving) by dynamically varying the delays associated with the different transducer segments such that the relative delays added to the more central transducer segments decrease as the focal point moves deeper into the body. Unfortunately, the need to provide a relatively large number of variable delays and circuitry to control these delays renders dynamic focusing an impractical expedient in many applications. The circuitry required therefor typically suffers one or more of the disadvantages of undue size, expense, complexity, and unreliability.
It is one object of the present invention to provide an imaging system and method including a dynamic focusing technique which overcomes disadvantage present in the prior art.